1. Field of the Invention
The present invention relates to an image transferring method using an intermediate transfer body, and an image forming apparatus for practicing the same. More particularly, the present invention is concerned with an image transferring method of the kind transferring a toner image from a photoconductive element or similar image carrier to a sheet or similar recording medium by way of an intermediate transfer body, an image transferring method of the kind transferring a toner image from a photoconductive element or similar image carrier to a sheet or similar recording medium by use of a belt capable of conveying the sheet, and a copier, printer, facsimile apparatus or similar image forming apparatus for practicing either one of the two methods.
2. Discussion of the Background
It is a common practice with an electrophotographic image forming apparatus, particularly a full-color image forming apparatus, to transfer a toner image from a photoconductive element to a sheet by two consecutive steps, i.e., a primary transfer step and a secondary transfer step. In the primary transfer step, consecutive toner images of different colors each are transferred from the photoconductive element to an intermediate transfer body implemented as a belt by way of example. In the secondary transfer step, the toner images transferred to the transfer body one above the other are collectively transferred to a sheet. For the primary transfer, an electric field is formed by a bias applied to one or both of two rollers over which the transfer body is passed. The two rollers are positioned at both sides of the photoconductive element. Alternatively, the two rollers may be connected to ground, in which case a bias will be applied to a contact member located at the center of a nip between the photoconductive element and the transfer body. The intermediate transfer body is often formed of a material having a medium volume resistivity (10.sup.8 .OMEGA.cm to 10.sup.13 .OMEGA.cm) or a medium surface resistivity (10.sup.7 .OMEGA. to 10.sup.12 .OMEGA.). With this kind of intermediate transfer body, it is possible to discharge a transfer charge applied from a charge applying means at the time of image transfer without resorting to a corona discharger or similar discharging means, or to reduce a required discharge output even when such discharging means is used.
However, the problem with the image forming apparatus of the type effecting the primary and secondary image transfer is that it is apt to blur the resulting image due to toner scattered around at the two image transfer steps. This kind of toner scattering varies with a transfer voltage and a transfer current.
Generally, the transfer current, transfer voltage and other transfer conditions are initially set before the shipment of the apparatus in such a manner as to minimize the above toner scattering while implementing the maximum toner transfer efficiency. However, the range of transfer conditions realizing both a high transfer efficiency and the satisfactory reduction of toner scattering is narrow. This, coupled with the fact that the optimal transfer conditions depend on the varying environmental conditions and the varying characteristics of the photoconductive element and intermediate transfer body, make it difficult to noticeably reduce the toner scattering. Specifically, when environmental conditions including temperature and humidity vary, the amount of charge to deposit on toner and the resistance of the transfer body also vary. Therefore, constant transfer conditions would lower the transfer efficiency or would bring about the toner scattering. Particularly, when the resistance of the transfer body decreases, the transfer voltage relatively exceeds its optimal value and aggravates the toner scattering due to, e.g., discharge occurring at an image transfer position.
To cope with the varying environmental conditions, it has been customary to provide the apparatus with a temperature sensor and a humidity sensor. Transfer conditions experimentally determined beforehand are selectively set up on the basis of the outputs of the above sensors, thereby compensating for a change in environment. On the other hand, a medium resistance material consisting of a resin and carbon black or similar conductive filler dispersed in the resin tends to lower its resistance with the elapse of time. As for an intermediate transfer body formed of such a medium resistance material, deterioration ascribable to aging is compensated for by the rough experiential estimation of the tendency of deterioration and varying the transfer conditions in accordance with the estimated tendency.
Japanese Patent Laid-Open Publication No. 4-45470 discloses an image forming apparatus of the type using a conveyor belt for image transfer and obviating pretransfer by causing a sheet and a photoconductive element to start contacting each other at a position upstream of an image transfer region. Japanese Patent Laid-Open Publication No. 4-186387 teaches an image forming apparatus of the type including a transfer drum and eliminating pretransfer by locating means for shielding an electric field at a position upstream of electric field forming means.
However, the above conventional image forming apparatuses each executes correction on the basis of experimental data or experiential data. Such apparatuses therefore cannot readily cope with operating conditions particular to the individual user or execute adequate correction.
When the intermediate transfer body or the transfer body for conveying a sheet is formed of a medium resistance material, the toner scattering at the time of image transfer is particularly noticeable. Specifically, when the intermediate transfer body is formed of a medium resistance material, the transfer charge applied from the charge applying means is capable of migrating even to the portions of the transfer body outside of the nip over which the image carrier and transfer body contact each other. As a result, a potential gradient and therefore an electric field is formed even on the surface of the intermediate transfer body outside of the nip. Particularly, an electric field formed at the inlet of the nip acts on the toner image carried on the image carrier at a position upstream of the nip in the direction of movement of the intermediate transfer body. As a result, the toner image is partly transferred from the image carrier to the intermediate transfer body before it reaches the nip (pretransfer), resulting in the fall of image quality. Further, in some kind of image forming apparatus, the undesirable electric field is formed at a position downstream of the nip and disturbs the toner image having been desirably transferred to the intermediate transfer body. This also brings about the toner scattering, irregular image density, local omission and other various kinds of defects.